You can "disclaim" your inheritance either under a will or an intestacy. When you disclaim you are deemed never to have taken your inheritance, and are not liable to inheritance tax.
If your brother pays you for disclaiming your share (consideration), then the amount he pays you is deemed to come from your aunt as an inheritance, and you may be liable to inheritance tax on that, depending on the usual rules.
Be careful when drawing up a disclaimer, there are rules out there about who gets what and where the disclaimed inheritance goes, so it might not always work out the way you want.
But you CAN do it, just make sure you get good legal advice to ensure you achieve the result you want.